1. Field of the Invention
This invention relates to optical drives used to read information from optical discs, and more particularly, to a focus/tracking method and system for use on an optical drive, which is capable of detecting both the focusing error and the tracking error of the pickup head to thereby control the focus/tracking of the same during read or write operation on an optical disc. This invention allows the focus/tracking method and system for the optical drive to be simplified in structural complexity, thereby saving manufacturing cost.
2. Description of Related Art
Pickup heads for optical discs must produce signals that indicate whether the optical stylus is in focus on the disc surface and the position of the optical stylus with respect to the information track besides just reading the coded information from the disc. As information is being recorded on the discs with ever-increasing density and in multiple layers, and as the number of styles of optical discs that a single pickup is expected to read is also increasing, more robust and versatile methods for producing these signals are called for.
It is well known that single-beam tracking performs better than three-beam tracking (which is commonly used for CD drives) when the tracks are spaced more closely together and there are multiple layers of information on the discs as, for example, in DVD discs. Single-beam tracking, which includes the methods of pushpull tracking, heterodyne tracking, and differential phase detection (DPD), also has the advantage over three-beam tracking of being generated directly from the disc information track rather than requiring critical alignment of tracking spots. Heterodyne and DPD tracking further have an advantage over pushpull tracking in that the pit depth which maximizes these tracking signals is the same as the depth which maximizes the information signal; whereas for pushpull tracking, its signal is at maximum at the pit depth which minimizes the information signal.
Multiple wavelength sources are required in pickups which are used to handle a wide variety of disc media. For example, red lasers of around 650 nm wavelength are required for reading DVD discs, while write-once CD-R media must be read using an infra-red laser with a wavelength around 780 nm. In order to avoid multiplying the number of components in the pickup, one set for each wavelength, a new means for generating the focus/tracking signals is needed which can be aligned properly for all of the wavelengths simultaneously. The differential spot-size detection method for focus-error signal generation is such a system that can be aligned for multiple wavelengths simultaneously but it is incompatible with heterodyne and DPD single-beam tracking methods.
Prior art for this invention includes a description of differential spot-size detection disclosed in Japanese Laid-Open Patent Document Number 63-229640 dated Sep. 26, 1988. The essential information processing scheme is reproduced in FIG. 1A, which includes a laser source 10, a holographic beamsplitter element 11, an objective lens 12, and a pair of 3-element photodetectors 16, 17. The holographic beamsplitter element 11 is used to divide the beam returning from the disc 13 into two beams 14 and 15, which are incident respectively on two 3-element photodetectors 16, 17. The holographic beamsplitter element 11 further has a focusing effect which causes the first beam to focus in front of one of the 3-element photodetectors and causes the second beam to focus behind the other 3-element photodetector. The spots on the photodetectors are diagrammed in FIGS. 1B-1D for various cases of the focus of the optical stylus on the information surface of the disc. The focus error signal (FES) indicating the focus error of the optical stylus with respect to the information surface in the disc is given by combining the electrical signals generated by the photodetector elements as follows:
FES=(SAxe2x80x3+SCxe2x80x3xe2x88x92SBxe2x80x3)xe2x88x92(SAxe2x80x2+SCxe2x80x2xe2x88x92SBxe2x80x2)
In the case shown in FIG. 1B, the stylus is focused behind the information surface which causes the spots from the two beams to have different sizes on their respective 3-element photodetectors and FES to be positive. For the case shown in FIG. 1C, the stylus is focused properly on the information surface, the spots from the two beams have the same size on their respective 3-element photodetectors, and FES=0. For the case shown in FIG. 1D, the stylus is focused in front of the information surface causing the spots from the two beams vary in a complementary manner to the case shown at the top and FES to be negative. For different wavelengths, the diffraction angles of the two beams from the holographic element vary, causing the spots to move along the photodetectors parallel to lines dividing the detector into three elements. This does not affect the resulting FES. Other prior art disclosing similar differential spot-size detection is found in U.S. Pat. No. 5,111,448 (May 1992). The drawback of these methods is that, since the complete beam area is incident on both of the 3-element photodetectors, there is no way to access the heterodyne and DPD tracking information which is embedded in an interference pattern in the beam.
An example of the interference pattern embedded in the beam is given in FIG. 2. A beam after experiencing diffraction from the information surface of the disc is shown centered on a coordinate system with quadrants labeled I, II, III, and IV. The arcs drawn within the main circular beam represent the overlap of the main circular beam and diffraction orders created by diffraction from the disc information surface. There is interference between these diffracted orders and the main beam. As the optical stylus moves on and off the information track, the intensity of these interference regions changes. The shaded areas indicate the interference regions that contribute to heterodyne and DPD tracking signals. Signals from each of the four quadrants must be available separately in order to generate these tracking signals. As stated above, the prior art for differential spot-size focus detection does not provide separate signals from these four quadrants and therefore cannot be used to generate these tracking signals. The astigmatic focus detection method is described in any introductory text to optical disc technology (e.g. A. B Marchant, Optical Recording, Addison Wesley Publishing, Reading, Mass., *990) and is a method which does provide separate access to the signals in each of the four quadrants. Moreover, for instance, U.S. Pat. No. 4,731,772 (Mar. 1988) uses a quadrant detector to provide separate signals from each of the four quadrants as shown in FIGS. 3A-3D. However, since the spot must remain centered on the quadrant photodetector, this approach is not tolerant of position shifts that will occur with multiple wavelength sources.
This invention uses a new method to combine some the best features of previously incompatible differential spot-size-detection focus-error and signal-beam tracking-error signal generation techniques to create a focus/tracking system that is well suited to multiple layer, high density and multiple wavelength optical disc systems while requiring a minimum number of components to implement.
This invention is compatible with all of the above-mentioned tracking methods, however the preferred embodiments utilize its special ability to produce heterodyne and DPD tracking signals in a multiple-wavelength system.
In accordance with the foregoing and other objectives of the present invention, a focus/tracking method and system for the pickup head of an optical drive is provided. The method of the invention includes the following steps of: generating a laser beam; focusing the laser beam on the optical disc; splitting the reflected light from the optical disc in half into a first half part and a second half part; guiding the first half part of the reflected light to a first optical axis while guiding the second half part of the reflected light to a second optical axis; at a fixed position on the first optical axis, detecting the first half part of the reflected light to thereby generating a first set of opto-electrical signals; at a fixed position on the second optical axis, detecting the second half part of the reflected light to thereby generating a second set of opto-electrical signals; and from the first and second set of opto-electrical signals, obtaining a focus error signal and a tracking error signal, the focus error signal being used for feedback control of the focusing of the laser beam until the laser beam is focused precisely on the optical disc, and the tracking error signal being used for feedback control of the tracking of the laser beam until the laser beam is spotted on the target data track.
The system of the invention includes the following constituent components: laser means for generating a laser beam of a specific wavelength; an objective lens, optically coupled to the laser means, for focusing the laser beam onto the optical disc; beam splitting means, optically coupled to the objective lens, capable of splitting the reflected light from the optical disc in half into a first half part and a second half part and directing the first half part of the reflected light to a first optical axis and the second half part of the reflected light to a second optical axis; a first photodetector disposed on the first optical axis, the first photodetector being formed with a plurality of light-sensitive elements capable of generating a first set of opto-electrical signals in response to the first half part of the reflected light spotted thereon; and a second photodetector disposed on the second optical axis, the second photodetector being formed with a plurality of light-sensitive elements capable of generating a second set of opto-electrical signals in response to the second half part of the reflected light spotted thereon. With the foregoing focus/tracking system, a focus error signal and a tracking error signal can be obtained from the first and second sets of opto-electrical signals from the first and second photodetectors. The focus error signal is used for feedback control of the focusing of the laser beam until the laser beam is focused precisely on the optical disc, while the tracking error signal is used for feedback control of the tracking of the laser beam until the laser beam is spotted on the target data track.
The foregoing focus/tracking method and system of the invention allows both of the focus error signal and the tracking error signal to be obtained from the same set of photodetectors, while still providing the benefits of maintaining alignment over multiple wavelengths and compatibility with single-beam tracking methods. The photodetectors used in the invention are each formed with a plurality of parallel light-sensitive elements. The light-sensitive elements of one photodetector are also in parallel with those on the other photodetector and perpendicular to the line which splits the light spot into two halves. This design scheme allows the photodetectors used in the invention to provide separate access to the signals in the four quadrants of the light spot as used in single-beam tracking methods. Furthermore, the elongated dimension of the light-sensitive elements is parallel to the direction light is deflected by the beamsplitter element allowing proper alignment between the beam and the light sensitive elements to be maintained even when the laser beam is changed in wavelength that causes a shift in the spotted location on these photodetectors. The invention is therefore suitable for use on an optical drive with a multiple-wavelength laser source.